Fe³⁺/Fe²⁺ Redox Couple Approaching 4 V in Li_2–x(Fe_1–yMn_y)P₂O₇ Pyrophosphate Cathodes

Li-metal pyrophosphates have been recently reported as novel polyanionic cathode materials with competent electrochemical properties. The current study presents a detailed analysis of inherent electrochemical properties of mixed-metal pyrophosphates, Li 2 (Fe 1– y Mn y )P 2 O 7, synthesized by an optimized solid-state route. They form a complete solid solution assuming a monoclinic framework with space group P 2 1 / c . The electrochemical analysis of these single-phase pyrophosphates shows absence of activity associated with Mn, where near-theoretical redox activity associated with Fe metal center was realized around 3.5 V. We noticed a closer look revealed the gradual substitution of Mn into parent Li 2 FeP 2 O 7 phase triggered a splitting of Fe 3+ /Fe 2+ redox peak and partial upshifting in Fe 3+ /Fe 2+ redox potentials nearing 4.0 V. Introduction of Mn into the pyrophosphate structure may stabilize the two distinct Fe 3+ /Fe 2+ redox reactions by Fe ions in octahedral and trigonal-bipyramidal sites. Increase of the Gibb’s free energy at charged state by introducing Li + –Fe 3+ and/or Li vacancy–Mn 2+ pairs can be the root cause behind redox upshift. The underlying electrochemical behavior has been examined to assess these mixed-metal pyrophosphates for usage in Li-ion batteries.